Edited as I was being 'special'. 
As an example download this user extension: https://market.sencha.com/extensions/login-panel
Then just rename it to .zip and have a look inside
A package.json file would look like this:
Code:
{
"name": "LoginPanel",
"type": "code",
"creator": "anonymous",
"summary": "Login Panel Extension",
"detailedDescription": "This extension allows easy creation of a commonly used login panel widget",
"version": "1.0.1",
"compatVersion": "1.0.0",
"format": "1",
"architect": {
"compatFrameworks": [
"touch20",
"touch21",
"touch22",
"touch23"
],
"classes": [{
"className": "sencha.login.LoginPanel",
"definition": "LoginPanel.js",
"js": [
"loginpanel.js"
],
"css": [
"loginpanel.css"
]
}]
},
"local": true,
"requires": []
}